A fuel cell is classified into different types according to the kind of electrolyte and the kind of electrode. Typical ones include an alkaline type, a phosphate type, a molten carbonate type, a solid electrolyte type, and a solid polymer type. Among them, more attention is paid to a solid polymer type fuel cell that can operate in a range from a low temperature (approximately −40° C.) to approximately 120° C., which, in recent years, has been increasingly developed and utilized in practice as a low-pollution power source for automobiles. The solid polymer type fuel cell is, according to the kind of fuel used, classified into a hydrogen-oxygen type fuel cell using hydrogen as fuel and a direct methanol type fuel cell using methanol as fuel. Then, uses of the hydrogen-oxygen fuel cell in the fields of power sources for vehicles and stationary power sources are under study, and the direct methanol type fuel cell is expected to be applied to uses for cellular phones and uses for portable power sources, distributed power supplies, and the like. For application to those uses, high performance and long-term durability are desired.
In the solid polymer fuel cell, a solid polymer electrolyte is sandwiched between an anode and a cathode, and a fuel is fed to the anode and oxygen or air is fed to the cathode to reduce the oxygen at the cathode so as to produce electricity. The fuel used is mainly hydrogen, methanol, or the like.
Conventionally, to increase the reaction rate of a fuel cell to enhance the energy conversion efficiency of the fuel cell, a catalyst-containing layer (hereinafter described also as “fuel cell catalyst layer”) has been provided on the surface of a cathode (oxygen electrode) and the surface of an anode (fuel electrode) of the fuel cell. In general, the catalyst is a noble metal. Among noble metals, platinum has been mainly used because of its stability at high potential and high activity. Additionally, carbon has conventionally been used as a carrier that supports the catalyst metal. Such a noble metal as platinum is used in large amount as a catalyst, resulting in significant increase in the cost of the fuel cell. This has been an obstacle in the development of fuel cells. In addition, due to limited deposits of noble metals and the like, studies are being conducted to reduce the amount of noble metal used.
In Patent Document 1, studies for reduction of the amount of a noble metal used have been made which include increasing effective surface area and mass activity by micronization and high dispersion of platinum, and alloying with other metals. However, under conditions of fuel cell operation, deterioration due to melting of platinum occurs. Accordingly, in order to maintain sufficient performance, there seems to be a limitation on the reduction of the amount of platinum used.
Meanwhile, to solve the cost problem, Patent Documents 2 and 3 reveal studies on oxycarbonitrides of transition metals such as tantalum and niobium, as alternatives to platinum. The abundance ratios of these transition metals in the earth are higher than that of platinum, and the metals are less expensive than platinum. Accordingly, the oxycarbonitrides of the transitional metals are expected as electrode catalysts for fuel cells. However, there has been a problem in which, as the electrode catalysts, such oxycarbonitrides have lower performance than platinum.
Patent Document 4 proposes a method in which in order to assist the performance of an oxycarbonitride of a transition metal, platinum is compounded with the oxycarbonitride thereof used as a carrier. The method employs a technique in which a carbide, a nitride, and an oxide of a transition metal such as niobium are mixed together and sintered at high temperature to produce a oxycarbonitride of niobium, and then the oxycarbonitride is used as a carrier to compound platinum therewith. The catalyst carrier has higher performance than in conventional platinum reduction methods and conventional transition metal oxycarbonitrides. However, despite that, the catalyst carrier has not yet reached a level of performance that can be used in practice.
In addition, the direct methanol type fuel cell has problems that, due to the crossover of methanol as liquid fuel, fuel utilization efficiency is degraded and the potential at the cathode is reduced, thereby resulting in significant degradation of energy conversion efficiency in the fuel cell. The methanol crossover is a phenomenon in which methanol moves from the anode to the cathode through a polymer electrolyte membrane. After reaching the cathode, the methanol is directly oxidized on the cathode catalyst surface, so that there occurs a problem that the potential at the cathode is lowered.
In general, the cathode catalyst of a direct methanol type fuel cell is a platinum catalyst. The platinum catalyst is highly active and highly stable. However, the platinum catalyst exhibits highly catalytic performance not only for the oxygen reduction reaction but also for the methanol oxidation reaction described above, thus promoting also the oxidation reaction of the liquid fuel reaching the cathode due to the crossover. As a result, an oxygen reduction potential at the cathode, together with an oxidation potential of the liquid fuel, forms a mixed potential, resulting in a significantly reduced level.
Additionally, in the direct methanol type fuel cell, the platinum catalyst is used in larger amount than in fuel cells using hydrogen, in order to promote reaction at the anode and also suppress potential reduction at the anode due to the fuel crossover. However, since platinum is expensive and platinum resources are limited, development of an alternative catalyst and significant reduction of use of platinum are strongly desired.
To suppress the crossover of fuel liquid in a direct methanol type fuel cell, there have been developed electrolyte membranes that hardly allow the transmission of liquid fuel and electrolyte membranes that do not allow the crossover of fuel liquid (for example, see Patent Documents 5 to 7).
However, it is extremely difficult for the electrolyte membranes described in Patent Documents 5 to 7 to significantly reduce the crossover of liquid fuel while maintaining high ionic conductivity and stability. Additionally, even with use of an electrolyte membrane suppressing the transmission of liquid fuel to some extent, the transmission of liquid fuel occurs more than a little along with the transmission of water in the electrolyte membrane, whereby the potential reduction at the cathode is inevitable.
There have also been reported catalysts that selectively perform only oxygen reduction without oxidizing methanol fuel which has reached the cathode due to methanol crossover.
Patent Documents 8 and 9 have reported Pd and Pd alloys having low methanol oxidation properties and high oxygen reduction activity, the Pd alloys being alloys with a noble metal such as Ru, Rh, Os, Ir, Pt, Au, or Ag (Patent Document 8) and alloy catalysts prepared by alloying with a transition metal such as Co, Cr, Ni, Mo, or W (Patent Document 9). However, although the catalyst metal described in Patent Document 8 is supported on a carbon carrier, high performance resulting from a synergistic effect of the carbon carrier and the Pd catalyst metal cannot be expected. In addition, the carbon carrier is easily corroded under a cathode atmosphere, whereby elution or desorption of the supported Pd catalyst metal may be accelerated. The Pd catalyst metal described in Patent Document 9 is produced by a sputtering method without using any carrier, so that it cannot be expected to increase a specific surface area of the catalyst metal by use of a carrier. Particularly, regarding the Pd catalyst metals described in Patent Documents 8 and 9 above, since studies have been made on oxygen reduction activity in a methanol-containing acidic aqueous solution electrolyte, it is difficult to confirm a cathode performance improvement effect by methanol crossover in an actual fuel cell. Therefore, cell evaluation is needed to be conducted by production of a membrane electrode assembly (MEA).
Meanwhile, Pd alloys (Pd—Sn, Pd—Au, Pd—Co, and Pd—WO3) described in Patent Document 10 are limited to a direct methanol type fuel cell using an alkaline electrolyte, and their methanol crossover suppression effect has not been confirmed in a direct methanol type fuel cell using an acidic solid electrolyte. In addition, evaluation has been conducted using a micronized Pd alloy, without using any carrier. However, for application to actual fuel cells, the pd alloy is needed to be supported on a carrier that can increase specific surface areas of the Pd alloy catalyst metals and catalyst utilization efficiency to conduct cell evaluation by production of a membrane electrode assembly (MEA).